Enlarged Broods Research Articles

Brood size manipulations reveal relationships among physiological performance, body condition and plumage colour in Northern Flicker Colaptes auratus nestlings

Visual signals of quality in offspring, such as plumage colour, should honestly advertise need and/or body condition, but links between nutritional status, physiological performance and the expression of colours are complex and poorly understood. We assess how food stress during rearing affected two physiological measures (T‐cell‐mediated immune function and corticosterone level in feathers: CORTf) and how these two variables were related to carotenoid and melanin coloration in Northern Flicker Colaptes auratus nestlings. We were also interested in how these two physiological measures were influenced by the sex of the nestling. We experimentally manipulated brood size to alter levels of food availability to nestlings during development. We measured carotenoid‐based colour (chroma and brightness) in wing feathers and the size of melanin spots on breast feathers. In agreement with our prediction, nestlings in the reduced brood treatment had better body condition and stronger immune responses than those in the control and brood enlargement treatments. This supports the hypothesis that immune responses are energetically costly. In contrast, CORTf was not related to nestling body condition or sex and was unaffected by brood size manipulation. Nestlings of both sexes with stronger T‐cell‐mediated immune responses had larger melanin spots but only males with higher immune responses also had brighter flight feathers. Feather brightness decreased with increasing CORTf levels. Our study is one of the few to examine the relationship between multiple physiological and plumage measures in nestlings and shows that plumage colour and immune function signalled body condition of nestlings, but that feather corticosterone levels did not.

Interacting effects of early dietary conditions and reproductive effort on the oxidative costs of reproduction.

The hypothesis that oxidative damage accumulation can mediate the trade-off between reproduction and lifespan has recently been questioned. However, in captive conditions, studies reporting no evidence in support of this hypothesis have usually provided easy access to food which may have mitigated the cost of reproduction. Here, I test the hypothesis that greater investment in reproduction should lead to oxidative damage accumulation and telomere loss in domestic zebra finches Taeniopygia guttata. Moreover, since the change or fluctuation in diet composition between early and late postnatal period can impair the ability to produce antioxidant defences in zebra finches, I also tested if early nutritional conditions (constant vs fluctuating early diet) influenced the magnitude of any subsequent costs of reproduction (e.g., oxidative damage and/or telomere shortening). In comparison to pairs with reduced broods, the birds that had to feed enlarged broods showed a higher level of oxidative DNA damage (8-OHdG), but brood size had no effect on telomeres. Fluctuating early diet composition reduced the capacity to maintain the activity of endogenous antioxidants (GPx), particularly when reproductive costs were increased (enlarged brood). The decline in GPx in birds feeding enlarged broods was accompanied by a change in bill colouration. This suggests that birds with lower endogenous antioxidant defences might have strategically increased the mobilization of antioxidants previously stored in other tissues (i.e., bill and liver) and thus, preventing an excessive accumulation of damage during reproduction.

Elevated reproduction does not affect telomere dynamics and oxidative stress.

Oxidative stress and telomere dynamics are considered to be powerful biomarkers quantifying a potential trade-off between current reproduction and self-maintenance. Recent studies confirmed the negative impact of elevated reproduction on telomeres, but the evidence for the cost of reproduction in terms of oxidative stress remains equivocal. In order to induce reproductive costs, we experimentally manipulated reproductive effort by increasing brood size in captive zebra finches (Taeniopygia guttata) and additionally challenged all birds by a low ambient temperature to facilitate detection of these costs. We were not able to show any negative effects of elevated reproductive effort on telomere dynamics and oxidative stress among parents, although brood enlargement was effective in terms of total mass and number of fledged young. Interestingly, irrespective of brood size treatment, we found a significant increase in antioxidant capacity at peak breeding while oxidative damage did not change with time. Our results may suggest that reproduction, instead of generating costs, may stimulate physiological functions promoting self-maintenance in terms of higher protection against free radicals. Possibly, opportunistic breeders such as zebra finches may not impede their future performance for the sake of current reproduction.Significance statementThis study interrogates a molecular background behind one of the most intriguing trade-offs that potentially occurs between self-maintenance and reproduction. We manipulated breeding effort in zebra finches to understand if the cost of reproduction can be mediated by telomere dynamics and oxidative stress. In our study system, we did not detect the direct reproductive costs in terms of parental oxidative damage and telomere loss; instead, these costs were paid by the offspring in terms of their inhibited growth rate. Moreover, we found that entering into the reproductive state strongly stimulated self-maintenance by increasing antioxidant capacity in parents. Our results emphasize that current reproductive success is not always prioritized over investment in body maintenance preventing the oxidative cost of reproduction.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2226-8) contains supplementary material, which is available to authorized users.

Brood size constrains the development of endothermy in blue tits.

Altricial birds are unable to maintain body temperature when exposed to low ambient temperatures during the first days after hatching. Thermoregulatory capacity begins to form as postnatal development progresses, and eventually nestlings become homeothermic. Several factors may influence this development at both the level of the individual and the level of the whole brood, but to our knowledge no studies have focused on the effect of brood size per se on the development of endothermy in individual nestlings. We performed cooling experiments on blue tit (Cyanistes caeruleus) nestlings in the field, to study how different experimental brood sizes affected the development of endothermy in individual nestlings and the thermal environment experienced by the whole brood in the nest. Nestlings from all experimental brood sizes showed a decrease in cooling rate as they grew older, but birds from reduced broods showed an earlier onset of endothermy compared with nestlings from enlarged and control broods. This difference manifested during early development and gradually disappeared as nestlings grew older. The thermal environment in the nests differed between treatments during nestling development, such that nest temperature in reduced broods was lower than that in enlarged broods during all days and during nights at the end of the experimental period. We suggest that the development of endothermy in blue tit nestlings is not ontogenetically fixed, but instead may vary according to differences in developmental, nutritional and thermal conditions as determined by brood size.

The oxidative cost of reproduction depends on early development oxidative stress and sex in a bird species.

In the early 2000s, a new component of the cost of reproduction was proposed: oxidative stress. Since then the oxidative cost of reproduction hypothesis has, however, received mixed support. Different arguments have been provided to explain this. Among them, the lack of a life-history perspective on most experimental tests was suggested. We manipulated the levels of a key intracellular antioxidant (glutathione) in captive zebra finches (Taeniopygia guttata) during a short period of early life and subsequently tested the oxidative cost of reproduction. Birds were allowed to mate freely in an outdoor aviary for several months. We repeatedly enlarged or reduced their broods to increase or reduce, respectively, breeding effort. Birds whose glutathione levels were reduced during growth showed higher erythrocyte resistance to free radical-induced haemolysis when forced to rear enlarged broods. This supports the hypothesis predicting the occurrence of developing programmes matching early and adult environmental conditions to improve fitness. Moreover, adult males rearing enlarged broods endured higher plasma levels of lipid oxidative damage than control males, whereas adult females showed the opposite trend. As most previous studies reporting non-significant or opposite results used females only, we also discuss some sex-related particularities that may contribute to explain unexpected results.

Sex-dependent responses to increased parental effort in the pied flycatcher

The optimal number of offspring for males and females may differ, as males and females invest differently in different aspects of reproduction. This creates potential for a sexual conflict leading to reduced residual reproductive value of the sex that experiences exploitation by the other sex. We experimentally investigated (by changing the brood size by one offspring) the effects of increased or decreased parental effort on the future local survival and breeding success of adult pied flycatchers (Ficedula hypoleuca), and on the future expression of male ornamentation. In addition, we studied how experimentally changing the brood size affected offspring condition and their recruitment. We show that females (but not males) rearing enlarged broods had lower local survival compared to those rearing reduced or control broods. This indicates a sexual conflict concerning optimal brood size. However, even though brood size manipulation did not affect the local survival of males, it did have an influence on the dorsal melanin coloration of young males. Young males rearing enlarged broods showed a stronger increase in plumage darkness from the current to the next breeding season than those rearing reduced or control broods. This suggests that stress experienced during reproduction might have carry-over effects that influence the complicated melanocortin system and lead to changes in the expression of melanin-based coloration. Alternatively, successful breeding might have stimulated young males to further invest in reproduction in the following season. Taken together these results indicate that both sexes are affected by brood demands, but in different ways.

Are prenatal maternal resources more important in competitive than in benign postnatal environments?

ABSTRACT According to theoretical models, the optimal solution of the life-history trade-off between the number and size of offspring depends on the quality of the environment. Offspring size should be more important for their fitness in more competitive environments. This idea was rarely experimentally tested in taxa with prolonged periods of parental postnatal care, such as in birds. Here we manipulated the offspring rearing environment by enlarging or reducing brood size. Enlarged broods suffered greater mortality rates and raised smaller fledglings. Egg size had a significant positive effect on fledging mass and length of tarsus and a nonsignificant effect on wing length. These effects were similar in enlarged-sized as well as reduced-sized broods. We only found a tendency for the predicted interaction between treatment and egg size in the case of nestling mass where egg size had a positive effect in enlarged broods but none in reduced broods. In contrast, in one year we found an opposite interaction ...

Observational field studies reveal wild birds responding to early-life stresses with resilience, plasticity, and intergenerational effects

ABSTRACT An extensive experimental literature documenting negative impacts of early stresses such as food deprivation, elevated corticosterone, and brood enlargement on numerous morphological, physiological, and behavioral traits of adult birds has left the impression of generalized developmental vulnerability to stress in birds. However, descriptive studies of wild populations, most of them recent, are indicating that natural stresses can sometimes elicit resilience or flexible variation in adult life history traits, and that such plasticity can neutralize impacts of stress on fitness. Experiments, almost entirely in captivity, were not designed to detect such effects, and most were not suited for this purpose in that treatments did not mimic natural stresses, effects were seldom measured over the lifespan, and fitness consequences were rarely assessed. Future research needs to take into account that early stresses can elicit positive developmental responses, that the direction of effects of stresses can...

Does coping style predict optimization? An experimental test in a wild passerine bird.

A number of studies have suggested that avian brood size is individually optimized. Yet, optimal reproductive decisions likely vary owing to among-individual differences in environmental sensitivity. Specifically, 'proactive' individuals who do not track environmental changes may be less able to produce optimal brood sizes than 'reactive' individuals who have more precise local environmental knowledge. To test this, we quantified exploratory behaviour (a proxy for proactivity) in a great tit (Parus major) population, manipulated brood sizes (reduced, control, enlarged) and evaluated whether individuals of dissimilar coping style differed in their level of optimization. If reactive females behaved optimally, any deviation from their original brood size should lower fitness, whereas this should not be the case for proactive females. Reactive females indeed performed best at their natural brood size, whereas proactive females performed best when raising an enlarged brood. These findings imply that proactive females produced sub-optimal brood sizes. We speculate that proactive females might (i) take decisions based on biased perception of their environment, (ii) face energetic constraints in offspring production and/or (iii) be more willing to invest into current reproduction when given the option. Our findings provide experimental evidence for coping style-related differences in optimal reproductive decisions and life-history strategies.

Physiological responses to increased brood size and ectoparasite infestation: Adult great tits favour self-maintenance

Different types of stressors trigger responses of different physiological systems, and these responses may contribute differentially to the maintenance of homeostasis, to trade-offs and the evolution of life-history traits. To manipulate two common stressors during reproduction, we infested half of the nests in a naturally breeding great tit population with ectoparasites and simultaneously manipulated brood size, using a 2×2 experimental design. Parents in this model species commonly compensate for ectoparasites by an increase in food provisioning. We assessed parental responses to these concurrent stressors by measuring several physiological stress parameters such as changes in metabolic rate, oxidative stress and expression of heat-shock proteins (Hsp), and explored how these stressors affect the trade-off between self-maintenance and reproduction. Neither flea infestation nor brood size manipulation affected adult metabolic rate, oxidative damage or Hsp levels. Furthermore, we found no interactive effect of the two treatments on adults. However, nestlings in infested nests had lower body mass and lower survival. Nestlings in enlarged broods were lighter and had lower survival, although parents of enlarged broods increased food provisioning rate. The findings suggest that adults favour maintenance of cellular homeostasis, and physiological equilibrium over current reproduction, and that the costs induced by both stressors, flea infestation and increased brood size, are carried by the offspring. It emphasizes the importance of self-maintenance over reproduction in life-history decisions, and more generally the need of including physiological traits for understanding the evolution of life-histories.

Addendum to: ‘Reproductive effort accelerates actuarial senescence in wild birds: an experimental study’

We recently showed that manipulation of reproductive effort over the entire lifespan modulated the rate of actuarial senescence in free-living jackdaws Corvus monedula (Boonekamp et al. 2014). We manipulated reproductive effort by giving birds either enlarged or reduced broods to rear (there were no unmanipulated birds), and quantified manipulation effects on parameters of the Gompertz equation fitted to our data using BaSTA (Colchero et al. 2012). We showed that the ageing rate was higher in birds rearing enlarged broods, while the baseline mortality rate was not affected. However, there was individual variation in the age at which birds entered the experiment, and colleagues noted that the way we incorporated the age of first manipulation in our statistical model could bias our Gompertz parameters estimates. We agree with this notion, and present an alternative model that addresses this point in the Supporting Information (S-AD). This model can generally be applied in settings where one aims to quantify differences between groups in mortality parameters, while there is variation in the age at which individuals are entered in the group. However, in our experiment, the variation in age at which birds were entered into the experiment was small, and as a consequence the alternative model yielded parameter estimates that are indistinguishable from our previous estimates (S-AD). Thus, applying the improved model did not change our results, but we recommend the new model when analysing mortality parameters in experiments where there is variation in the age at which individuals are entered into the experiment.

Sex- and melanism-specific variations in the oxidative status of adult tawny owls in response to manipulated reproductive effort.

Oxidative stress, determined by the balance between the production of damaging reactive oxygen species (ROS) and antioxidant defences, is hypothesized to play an important role in shaping the cost of reproduction and life history trade-offs. To test this hypothesis, we manipulated reproductive effort in 94 breeding pairs of tawny owls (Strix aluco) to investigate the sex- and melanism-specific effects on markers of oxidative stress in red blood cells (RBCs). This colour polymorphic bird species shows sex-specific division of labour and melanism-specific history strategies. Brood sizes at hatching were experimentally enlarged or reduced to increase or decrease reproductive effort, respectively. We obtained an integrative measure of the oxidative balance by measuring ROS production by RBCs, intracellular antioxidant glutathione levels and membrane resistance to ROS. We found that light melanic males (the sex undertaking offspring food provisioning) produced more ROS than darker conspecifics, but only when rearing an enlarged brood. In both sexes, light melanic individuals had also a larger pool of intracellular antioxidant glutathione than darker owls under relaxed reproductive conditions (i.e. reduced brood), but not when investing substantial effort in current reproduction (enlarged brood). Finally, resistance to oxidative stress was differently affected by the brood size manipulation experiment in males and females independently of their plumage coloration. Altogether, our results support the hypothesis that reproductive effort can alter the oxidative balance in a sex- and colour-specific way. This further emphasizes the close link between melanin-based coloration and life history strategies.

Northern Flickers increase provisioning rates to raise more but poorer quality offspring when given experimentally enlarged broods

ABSTRACT Brood enlargement experiments have been conducted in several species of birds to investigate how parents of both sexes adjust their investment in the current breeding attempt. We studied parental feeding effort in the Northern Flicker (Colaptes auratus), a species with partially reversed sex roles where males invest more in parental care than females and in which there is facultative polyandry and no extra-pair young. By experimentally manipulating brood sizes to be either larger or smaller by about 40%, we tested the flexibility of provisioning responses by each sex and whether the upper limit to provisioning corresponded to the maximum clutch size in the population. Both male and female parents increased feeding rates to enlarged broods, but per-nestling provisioning declined so that they fledged lighter nestlings with shorter wings. Mortality in reduced broods was lower than in control broods, but there was no difference in the incidence of mortality between control and enlarged broods. Parent...

Experimentally increased reproductive effort alters telomere length in the blue tit (Cyanistes caeruleus).

Telomeres have recently been suggested to play important role in ageing and are considered to be a reliable ageing biomarkers. The life history theory predicts that costs of reproduction should be expressed in terms of accelerated senescence, and some empirical studies do confirm such presumption. Thus, a link between reproductive effort and telomere dynamics should be anticipated. Recent studies have indeed demonstrated that reproduction may trigger telomere loss, but actual impact of reproductive effort has not received adequate attention in experimental studies. Here, we experimentally manipulated reproductive effort by increasing the brood size in the wild blue tit (Cyanistes caeruleus). We show that parents attending enlarged broods experienced larger yearly telomere decay in comparison to control birds attending unaltered broods. In addition, we demonstrate that the change in telomere length differs between sexes, but this effect was independent from our treatment. To our knowledge, this is the first experimental study in the wild revealing that telomere dynamics may be linked to reproductive effort. Thus, telomere shortening may constitute one of the potential proximate mechanisms mediating the costs of reproduction.

Resting and daily energy expenditures during reproduction are adjusted in opposite directions in free‐living birds

Summary Reproduction is energetically expensive, and daily energy expenditure (DEE) often peaks during the period of rearing young. The ‘potentiation’ hypothesis predicts that high DEE needs to be sustained by a corresponding up‐regulation of metabolic machinery; thus, a concomitant increase in the resting metabolic rate (RMR) is expected. Alternatively, the ‘compensation’ hypothesis predicts that DEE and RMR are regulated independently and animals may maintain low RMR to maximize the energy available for reproduction. This might particularly be the case if DEE was limited, for example, by extrinsic food supply or intrinsic physiological factors. We tested these hypotheses in free‐living seabirds by manipulating their energy demands (experimentally reduced or increased brood size) and supplies (providing supplemental food), and simultaneously measuring their DEE and RMR (by the doubly labelled water method and an indirect hormonal proxy, respectively). In support of the ‘compensation’ hypothesis, metabolic rates were adjusted independently and in opposite directions with an increase in DEE and a decrease in the hormonal proxy for RMR in individuals rearing young compared to birds with removed broods. Energy expenditure of unfed birds with chicks appeared to be limited as experimental brood enlargement did not cause an increase in DEE. Supplemental feeding did not allow DEE to exceed this apparent limitation. We propose that a reduction in the resting metabolism is a strategy to increase allocation of energy to reproduction when DEE is constrained and that this constraint is unlikely to be related to food supply.

Brood size manipulations in a spatially and temporally varying environment: male Tengmalm's owls pass increased reproductive costs to offspring.

A key tool used to assess reproductive trade-offs in birds is brood size manipulation (BSM) experiments. Most BSM studies have examined the influence on short-term measures of reproductive output. Seldom evaluated are the effects on long-term fitness proxies under temporally or spatially varying environments. Unpredictable environments may affect reproductive trade-offs by altering the value of the brood or hampering optimization of reproductive effort. We reduced or enlarged broods of 140 male Tengmalm's owls Aegolius funereus by one chick during their first lifetime reproductive event. Males differed in age and bred in environments that varied in quality spatially (habitat structure) and temporally (abundance of main food). We measured the short-term (nestling number and condition) and long-term fitness proxies (survival, lifetime fledgling and recruits produced) until all experimental males disappeared from the population. BSMs did not affect fledgling number or condition, but in enlarged broods, offspring condition was lower in territories with a high proportion of agricultural fields. Importantly, no obvious impacts on long-term fitness proxies emerged; lifetime fledgling and recruit production of males did not differ between the BSM treatments. Thus, the primary caregiver (i.e. Tengmalm's owl males) passed increased reproductive costs to their offspring, which is in agreement with other studies investigating intergenerational reproductive trade-offs in species of intermediate lifespan. Reluctance to accept increased current reproductive costs in these systems highlights the potential for sexual conflict in bi-parental care systems in which one of the pair is the primary caregiver.

Responses by Central‐Place Foragers to Manipulations of Brood Size: Parent Flickers Respond to Proximate Cues but do not Increase Work Rate

AbstractManipulations of brood size measure the willingness or ability of parents to invest in offspring and different reproductive roles may lead to differences in feeding effort between the sexes. Parental investment in birds is usually assessed by quantifying feeding rates, but this provides an incomplete picture of parental effort because it fails to account for how parents collect food on the landscape. We studied northern flickers (Colaptes auratus), a woodpecker in which males provide the majority of parental care and used a repeated measures design and short‐term (24 h) brood enlargements (N = 35) and reductions (N = 27) to assess effects of treatment on feeding rates to nestlings and parental foraging behaviour. Parents of enlarged broods did not significantly increase feeding rate, resulting in a decline in nestling mass. Parents of reduced broods decreased their feeding rates by 84%, but increased per capita feeding rates, resulting in nestling mass gain. The variation in feeding rates to enlarged broods was not influenced by feather corticosterone, body condition, feather re‐growth rate or mass change between the incubation and nestling periods. Foraging pattern on the landscape remained the same during the enlarged treatment for both sexes. We conclude that flickers respond to proximate cues in brood demands, but do not increase feeding rates to enlarged broods, at least in the short term. A literature review suggested that this lack of response is atypical for short‐lived species. We hypothesize that parents in species with large home ranges and long nestling periods face energetic limitations that constrain their ability to respond to enlarged broods. We encourage future studies to assess foraging behaviour on the landscape to document important trade‐offs for parents such as predation risk and energy expenditure while feeding offspring.

Nestling rearing is antioxidant demanding in female barn swallows (Hirundo rustica).

Reproduction is a demanding activity, since organisms must produce and, in some cases, protect and provision their progeny. Hence, a central tenet of life-history theory predicts that parents have to trade parental care against body maintenance. One physiological cost thought to be particularly important as a modulator of such trade-offs is oxidative stress. However, evidence in favour of the hypothesis of an oxidative cost of reproduction is contradictory. In this study, we manipulated the brood size of wild barn swallows Hirundo rustica soon after hatching of their nestlings to test whether an increase in nestling rearing effort translates into an increased oxidative damage and a decreased antioxidant protection at the end of the nestling rearing period. We found that, while plasma oxidative damage was unaffected by brood size enlargement, females rearing enlarged broods showed a decrease in plasma non-enzymatic antioxidants during the nestling rearing period. This was not the case among females rearing reduced broods and among males assigned to either treatment. Moreover, individuals with higher plasma oxidative damage soon after the brood size manipulation had lower plasma non-enzymatic antioxidants at the end of the nestling rearing period, suggesting that non-enzymatic antioxidants were depleted to buffer the negative effects of high oxidative damage. Our findings point to antioxidant depletion as a potential mechanism mediating the cost of reproduction among female birds.

Insights into life history theory: a brood size manipulation on a southern hemisphere species, Tachycineta leucorrhoa, reveals a fast pace of life

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive effort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white‐rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding effort in enlarged broods, at least enough to maintain nestlings’ development/growth. In addition, adults decreased the number of visits to the nest (without having a negative effect on nestlings) in reduced broods. We did not detect differences in fledging success among treatments, suggesting there were no differences in nestlings’ survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. This is one of the first studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.

The adaptive function of hatching asynchrony: an experimental study in great tits

In many bird species offspring hatch over hours or days, which leads to an age and size hierarchy within broods. The function of hatching asynchrony is much debated, and it has been suggested that the induced size hierarchy among offspring may be an adaptive maternal mechanism for maximizing reproductive output under variable environmental conditions. The best known hypothesis to explain the adaptive value of hatching asynchrony, the ‘brood reduction’ hypothesis, holds that a size hierarchy among offspring allows for a quick adaptive adjustment of brood size to unpredictable feeding conditions and thus benefits parents. To test the consequences of hatching asynchrony on offspring growth and food provisioning we experimentally manipulated the onset of incubation of eggs within broods of great tits, Parus major , to induce either synchronous or asynchronous hatching, and then manipulated brood size after hatching to simulate favourable, control or harsh conditions. We did not find a difference in offspring mortality between asynchronous and synchronous broods under any of these conditions. In harsh conditions, nestling mass and size were reduced in asynchronous broods compared to synchronous ones. The opposite pattern occurred under control conditions. Although our results showed that induced hatching asynchrony alters offspring phenotype, they do not provide strong support for the brood reduction hypothesis as a mechanism to explain hatching asynchrony. • We tested the adaptive value of hatching asynchrony in great tits. • We induced either synchronous or asynchronous hatching. • We simultaneously manipulated the rearing conditions by changing brood size. • Nestling mortality did not differ between asynchronous and synchronous broods. • In enlarged broods, nestlings in asynchronous broods were lighter and smaller. • Our results do not support the brood reduction hypothesis.